Apparatus for improving metal structure



Oct. 27, 1964 c. B. CRINER 3,153,820

APPARATUS FOR IMPROVING METAL STRUCTURE Filed Oct. 9, 1961 2 Sheets-Sheet l INVENTOR 29 Cheg'lesz. C'rzlner ATTORN EYS Oct. 27, 1964 c. B. CRINER 3,153,820

APPARATUS FOR IMPROVING METAL STRUCTURE Filed Oct. 9, 1961 2 Sheets-Sheet 2 INVENTOR C/zcglesfi. Urine/- Y ATTORNEYS United States Patent hhury, Qonn.) Filed Get. 9, 1961, No. 143,720 5 Claims. (til. 22-572) This invention relates to apparatus for improving the grain structure and the physical and chemical homogeneity of metallic materials by controlling thei solidification from the molten state. The resultant material has minrnum segregation and inhomogeneity, whereby substantially improved physical properties, such as tensile strength and yield strength are achieved. The invention is particularly suited for the production of metals, and will be described in that context.

Casting molten metal as ingots offers the best means presently known for solidifying molten metal and economically forming it for subsequent shaping, as by rolling or forging. in general, the molten metal in an ingot mold has different temperatures, being cooler at the surfaces where the mold removes heat. in addition, the melt comprises dferent chemical constituents, such as alloying metals and impurities. These constituents have different freezing temperatures, just as water and alcohol, for example, have different freezing temperatures.

solidification of the metal begins when a nucleus is formed, i.e., when enough atoms come together in the proper arrangement to constitute a crystal capable of growing. The crystallization begins in the cooler regions of the melt and starts with the constituents having the highest freezing temperatures. The nucleus grows when additional atoms join with it in selected arrangements. Constituents having lower freezing temperatures do not crystallize but remain in the molten phase, being segregated from the nucleating crystal until they cool sufficiently to crystallize. Thus, the chemical compositions of the growing crystals change, initially comprising a high concentration of constituents having high freezing temperatures. As the temperature decreases, difierent chemical constituents, having progressively decreasing freezing temperatures, nucleate on the growing crystal. This segregation results in the ingot having an inhomogeneous chemical composition that persists through subsequent worlting and shaping to the final product.

The crystals do not grow uniformly in all directions but, rather, grow in a tree- 'ke fashion in preferred directions forming dendrites. Generally, dendritic crystals grow in long, parallel columns, perpendicular to the ingot surfaces, penetrating to the center of the ingot as it cools. The columnar crystals result in the solid metal being macrosegregated, from the surface to the center of the ingot, and the individual crystals are macrosegregated due to the changing chemical compositions that freeze at different temperatures, as explained above.

The zones between adjacent crystals freeze last, forming cell boundaries. The boundary zones drastically aifect the physical properties and behaviors of the metal. For example, the crystal imperfections and segregation that occur in ingots manufactured according to prior techniques are believed to cause metals to have actual strengths that are substantially less than the theoretical strengths, the strength discrepancy reportedly estimated as ranging from one one-hundredth to one ten-thousandth of the calculated theoretical value.

Heat treating to improve homogeneity, and cold working to reduce grain size, only improve slightly the metal structure and segregation frozen into the ingot; they do not avoid it. Thus these prior art techniques have, at best, only slightly improved the strength of metals, as

compared to the theoretical values.

EJ53321 Fatented Get. 27, T96 3- l'n the prior art, various techniques have been devised to improve the stren ths of metals by accelerating ingot cooling and by reducing differences in cooling rates, to minimize internal stresses. in addition, attempts have een made to cast ingots at lower temperatures to reduce the crystal size and to use smaller ingots, which, however, results in production inefiiciencies.

A more recent technique, magnetic stirring, is directed to improving the metal structure prior to and during the initial casting process. Magnetic stirring, according to one prior technique, is affected by passing an electric current through a single toroidal coil that surrounds the metal in an electric arc furnace. The interaction of the arc current and the magnetic field produced by the coil develops forces that stir the liquid metal throughout the furnace. After the metal is well stirred, it is poured into conventional ingot molds. Although some improvements in ingot structure have been realized, they are relatively limited.

United States Patent No. 1,978,222 describes another prior technique for electromagnetically stirring molten metal, whereby an electric current is passed longitudinally through the metal in an ingot mold. The entire ingot is subjected to the resultant electromagnetic forces, and the current is generally maintained until the entire ingot has solidified. However, the technique does not provide sufficient control and selective variation of the agitating forces controlling the solidification to achieve homogeneity in the frozen metal.

Accordingly, it is a principal object of the present invention to provide cast and molded materials having improved characteristics and qualities, thereby to more closely approach their theoretical strengths.

Another object of the invention is to provide improved apparatus for casting materials, particularly metals, with a more homogeneous structure than heretofore available. Thus, it is an object that the apparatus will produce materials having homogeneous and uniform crystals. Furthermore, the apparatus produces cast and molded materials having a substantially uniform distribution of chemical constituents.

A further object of the invention is to control the solidification of molten metal to improve the homogeneity of the resultant solid metal.

Yet another object of the present invention is to substantially eliminate the need for hot and cold working in the production of metals.

With the growing trend toward casting metals on a continuous basis, the cross-section of the cast product more nearly approximates that of the final product, so that del ciencies, dislocations and voids will become even more critical than in the past, because breakdown rolling, both hot and cold, cannot be incorporated to the degree utilized in the former practices using large ingots.

In the continuous casting practices, there is an inherent tendency toward bridging of adjacent dendritic crystals, creating voids or entrapping gases or lower-freezing-point liquid that would ideally solidify in a different fashion than in this captive condition. This invention substantially minimizes, if not prevents, bridging such that entrapped areas are substantially avoided and a more homogeneous final product results. in addition to chemical inhomogeneities, these bridging areas are sources of shrinkage and microcraclrs that are further deleterious to sound metal.

A further object of the invention is to provide improved apparatus for treating molten metal by magnetic agitation.

A still further object is to provide a highly efiicient apparatus for agitating molten metal.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangements of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a sectional side elevation view of direct chill-casting metal-producing apparatus embodying the invention;

FIGURE 2 is a perspective pictorial representation of electromagnetic agitators disposed for stirring metal ac cording to the present invention;

FIGURE 3 is a perspective pictorial representation of a combination of electromagnetic and mechanical agitators for stirring metal according to the present invention;

FIGURE 4 is a sectional top plan view of the direct chill-casting apparatus, taken along line 4-4 of FIG- URE 1;

FIGURE 5 is a sectional side elevation view of waterdip metal-casting apparatus embodying the present invention; and

FIGURE 6 is a sectional side elevation view of ingotcasting apparatus embodying the invention.

In general, according to the present invention, control of the solidification of molten metals is initiated as soon as the metal is poured into an ingot mold, preferably prior to the first nucleation, with a plurality of mechanical, and electromagnetic agitators that homogenize the melt. The agitators focus the homogenizing treatment, or agitation, in a narrow region that includes and extends above the freezing zone adjacent the solidified portion of the ingot metal. The agitators are preferably operated at varying rates to obtain the grain structure desired in the finished product.

The agitation penetrates throughout the narrow, transverse region on which it is focused and is preferably substantially uniform across the ingot. Thus, it is distinguished from surface stirring or violent turbulence that disrupts the crystallization of the metal.

The present controlled and focused agitation, or homogenizing treatment, prevents the formation of large crystals and minimizes segregation within the ingot. In addition, temperature differences are minimized in the freezing zone. By thus controlling the crystallization, the nuclei are arranged for maximum stability to more closely approximate the theoretical metal structure than heretofore possible. Furthermore, by focusing the agitation on the freezing zone, rather than throughout the ingot, the stirring is substantially more efficient than with prior techniques.

Referring now to FIGURES l and 4, the present invention is suitable for use with direct chill-casting apparatus in which molten metal, indicated at 10, is poured from the nozzle 12 of a ladle (not shown) into an ingot mold 14 that has a movable bottom 16 and stationary side walls 18. The metal solidifies in the mold 14 and the frozen portion 20 thereof descends as the bottom 16 is moved downward, as indicated by the arrow 22.

A freezing zone indicated at 24 develops between the 'solid portion 20 and the molten metal 10. The bottom 16 is preferably moved downward at the same rate that the solid portion 20 grows, so that the freezing zone 24 remains substantially stationary with respect to the walls 18 of the ingot mold 14 e A system of agitators 26 is disposed around the mold 14 substantially horizontally. adjacent to the freezing zone 24. The agitators 26 are connected to a driver 28, generally electronic, that. controls their operation. The agitators may be mounted on a support that is preferably constructed to allow them to be positioned vertically with respect to the mold 14.

Referring now to FIGURES 1 and 2, the system of agitators 26 may comprise a plurality of individual electromagnets 29, as illustrated in FIGURE 2, disposed in a peripheral path around the mold 14 (not shown in FIGURE 3) adjacent the freezing zone 24 of the molten metal. FIGURE 2 shows the limited zoned action; there is no disturbance of material which is substantially crystallized. Although a single row of agitators is shown in FIGURE 2, a second row may be arranged adjacent to the first row. Similarly, the electromagnets 29 should be generally horizontal, as shown, to produce the desired magnetic field configuration.

The electronic driver 28 selectively energizes the electromagnets 2% to generate magnetic fields that induce eddy currents in the molten metal. Either alternating or direct current may be used to energize the electromagnets 29. The agitators are operated to orient and focus the eddy currents in the freezing zone and to agitate the molten metal so that the nuclei therein are oriented in a stable arrangement. Thus the electromagnets 29 may be operated continuously and/or successively, and at varying frequencies and/ or phase relationships, to develop agitating forces of different intensities. By providing a plurality of independently operable agitators, different modes of stirring may be combined to orient the nuclei for optimum crystallization. The resultant agitation substantially prevents segregation and allows the metal to solidify in a substantially homogeneous condition.

Some of the electromagnetic agitators 26 may be energized at a frequency sufficient to produce induction heating in the ingot, to further eliminate cold spots and to ensure that crystallization occurs primarily in the relatively narrow zone immediately below the region exposed to induction heating. Induction heating is particularly useful in the upper arrays of agitation shown in FIGURE 6, to retard premature solidification, referred to in the art as pipe formation.

The agitators 26 may also be mechanical, such as sonic, ultrasonic or acoustical transducers, disposed in a pcripheral band adjacent to the freezing zone and individually actuated to control the crystallization for the most stable configuration. As shown in FIGURE 3, both electromagnets 29 and mechanical agitators 31 may be combined either symmetrically or otherwise to control the crystallization of the molten metal 10. Additional agitators may be arranged in an outer band to further control the agitation or to achieve more varied modes of agitation.

The effect of agitating the molten metal as described above is to stir the melt in a relatively narrow transverse region, including the freezing zone, whereby dendritic crystallization and segregation are minimized.

A coolant, such as water or oil, may be circulated through the agitators 26 by way of a cooling system, a portion of which is indicated in FIGURE 1 at 33, to remove heat developed in the agitators. Since the metal is initially cast under conditions that promote homogeneity, any dislocations, strains, discontinuities, and other defects are minimized. Hot and cold working processes may thus be eliminated or substantially reduced during the production of high-grade metals, to efiect savings in time and equipment. This is particularly applicable to the continuous casting of metal wherein ingot thickness does not permit homogenization through breakdown and cold working techniques that have been so utilized in the prior art.

Referring now to FIGURE 5, the present stirring apparatus may be combined with specialized casting practices for chilling a metal ingot in a bath of water or a similar coolant. The molten metal 10 is contained in an ingot mold 32 that is gradually lowered into a water bath 34 at a rate approximating that of metal solidification at the freezing zone 24. Agitators 26, supported on a mount 36, are disposed peripherally, similar to the agitators of FIGURE 4, adjacent the freezing zone 24 and are oper ated as described above to agitate the melt. It is contemplated that the agitation is focused in a narrow zone substantially perpendicular to the direction in which the melt solidifies. Although solidification progresses from the bottom to the top of the ingot and from the sides to the center, so that the freezing zone normally has a downwardly curved shape, the focused agitation substantially flattens the freezing zone to a substantially horizontal region.

A conventional ingot mold 37, having the big end down, is shown in FIGURE 6 with the agitators 26 disposed around it on a support 38. in this ingot mold, with the present invention, the solidification zone is essentially two regions, one confined to a thin peripheral region 24a adjacent the inner wall of the mold 3'7, which is progressively merged in a second substantially horizontal transverse zone 24!), which rises toward the top of mold 37 as solidification proceeds. Agitation in the entire mold would be activated from the beginning of solidification, i.e., as soon as the mold is filled. As the lower zone 24b rises through the mold, agitators below this rising level may be inactivated. This process minimizes or avoids pipe formation and thereby eliminates one of the most costly sources of defect in casting practices utilizing the fixed ingot mold technique illustrated in FIGURE 6. It should again be noted that in each of the foregoing embodiments, a plurality of agitators 26, as shown in FIGURES 2, 3, and 4, is disposed in each row thereof around the body of molten metal.

In summary, apparatus for homogenizing and selectively controlling agitation of a relatively narrow zone of metal has been described. The zone of agitation includes the liquidus zone, wherein the metallic nuclei crystallize. The agitation substantially increases the homogeneity of the cast metal, substantially eliminating segregation and dendritic crystallization, so that the properties of the ingot metal more closely approximate theoretical values.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in carrying out the above methods and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. Apparatus for treating a solidifying mass of molten metal, said apparatus comprising in combination a plurality of independently-operable external agitators including both electro-magnets and vibratory electromechanical transducers disposed outside said mass of molten metal and spaced around and substantially adjacent to the freezing zone of the metal, said agitators producing when energized a plurality of agitating force fields focused within said mass, an agitator-energizing source connected with said agitators, said source selectively energizing said agitators nonuniform-1y, so that the molten metal in the freezing zone is agitated by the agitating force fields to control properties of the resultant solid metal.

2. Apparatus for treating a solidifying mass of molten metal, said apparatus comprising in combination a plurality of independently-operable external agitators disposed outside said mass of molten metal and spaced around and substantially adjacent to the freezing zone of the metal, said agitators producing when energized a plurality of agitating force fields focused within said mass, an agitator-energizing source connected with said agitators, said source selectively energizing said agitators nonunifornfiy, and movable support means for guiding and advancing the mass of molten metal at a speed selected to maintain the progressively advancing freezing zone within the focused force fields, so that the molten metal in the freezing zone is agitated by the agitating force fields to control properties of the resultant sol-id metal.

3. The combination defined in claim 2 in which said agitators are disposed in a plurality of peripheral bands around said mass of molten metal, and further comprising a separate selectively-phased agitator-energizing source connected to the agitators in each band to focus the resultant agitating force fields substantially exclusively in the freezing zone of the metal.

4. The combination defined in claim 2 in which at least one of said agitators is constructed and operated to cause induction heating of the metal in said focused force fields.

5. The combination defined in claim 2 in which said agitator-energizing source produces fluctuations in said agitating force fields having predetermined frequency and amplitude characteristics with at least one of said characteristics of at least one of said force fields being independently variable according to a preselected pattern, whereby said plurality of force fields includes differing force fields produced in dilferent portions of said solidifying mass.

References Cited in the file of this patent UNITED STATES PATENTS 2,963,758 Pestel Dec. 13, 1960 3,045,302 Patton July 24, 1962 FOREIGN PATENTS 528,354 Canada July 31, 1956 752,271 Great Britain July 11, 1956 OTHER REFERENCES Battelle Technical Review, vol. 5, No. 4, April '1956, pages 9-13. 

1. APPARATUS FOR TREATING A SOLIDIFYING MASS OF MOLTEN METAL, SAID APPARATUS COMPRISING IN COMBINATION A PLURALITY OF INDEPENDENTLY-OPERABLE EXTERNAL AGITATORS INCLUDING BOTH ELECTRO-MAGNETS AND VIBRATORY ELECTROMECHANICAL TRANSDUCERS DISPOSED OUTSIDE SAID MASS OF MOLTEN METAL AND SPACED AROUND AND SUBSTANTIALLY ADJACENT TO THE FREEZING ZONE OF THE METAL, SAID AGITATORS PRODUCING WHEN ENERGIZED A PLURALITY OF AGITATING FORCE FIELDS FOCUSED WITHIN SAID MASS, AN AGITATOR-ENERGIZING SOURCE CONNECTED WITH SAID AGITATORS, SAID SOURCE SELECTIVELY ENERGIZING SAID AGITATORS NONUNIFORMLY, SO THAT THE MOTLEN METAL IN THE FREEZING ZONE IS AGITATED BY THE AGITATING FORCE FIELDS TO CONTROL PROPERTIES OF THE RESULTANT SOLID METAL. 